Reducing agents for silver morphology control

ABSTRACT

A method comprising providing at least one reducing agent comprising at least one phenol group, the at least one reducing agent not also comprising a halogen atom, and reducing at least one silver ion to at least one silver nanowire in a reaction mixture comprising the at least one reducing agent. Exemplary reducing agents are 3,4-dihydroxybenzotriazole, 2,2′-isobutylidene-bis-(4,6-dimethyl-phenol), and tannic acid.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/020,431, filed Jul. 3, 2014, entitled “REDUCING AGENTS FOR SILVERMORPHOLOGY CONTROL,” which is hereby incorporated by reference in itsentirety.

BACKGROUND

The general preparation of silver nanowires (10-200 aspect ratios) fromsilver ions is known. See, for example, Y. Xia, Y. Xiong, B. Lim, S. E.Skrabalak, Angew. Chem. Int. Ed., 2009, 48, 60; J. Jiu, K. Murai, D.Kim, K. Kim, K. Suganuma, Mat. Chem. & Phys., 2009, 114, 333; U.S.patent publication no. 2013/0192423 to Yang et al.; U.S. Pat. No.7,922,787 to Wang et al.; and U.S. Pat. No. 8,613,888 to Whitcomb etal., all of which are hereby incorporated by reference in their entiretyherein. Such preparation methods typically involve the use of a reducingagent to reduce silver ions and form nanoscale silver seed particles,which may lead to the formation of silver nanowires. Y. Xia, Y. Xiong,B. Lim, S. E. Skrabalak, Angew. Chem. Int. Ed., 2009, 48, 60. In someprocesses, silver nanowire growth may be promoted by metallic seeds fromhigh temperature preparations, such as palladium or platinum. Y. C. Lu,K. S. Chou, Nanotechnology, 2010, 215707, 6 pages, which is herebyincorporated by reference in their entirety herein.

Some relatively strong reducing agents, such as, for example, ascorbicacid, sodium borohydride, or alkylamines in toluene have been used. See,for example, Mehdi Jalali-Heravi, Hossein Robatjazi, HeshmatollahEbrahimi-Najafabadi, Physicochem. Eng. Aspects, 2012, 393, 46; M. S.Bakshi, J. Nanosci. Nanotechn. 2010, 10, 1757; and H. Hiramatsu, F. E.Osterloh, Chem. Mater., 2004, 16, 13, 2509, all of which are herebyincorporated by reference in their entirety herein. Other less strongreducing agents, such as, for example, reducing agents thermallygenerated from ethylene glycol, have been used. J P Lagier, B. Blin, B.Beaudoin, M. Figlarz, Solid State Ionics, 1989, 32/33, 198; S. E.Skrabalak, B J Wiley, M. Kim, E V Formo, Y. Xia. Nano Letters, 2008,8(7), 2077-81; Silvert P-Y, et al. J. Mater. Chem., 1997, 7, 293-9; andSilvert P-Y, et al., J. Mater. Chem., 1996, 6, 573-7, all of which arehereby incorporated by reference in their entirety herein.

In some cases, large crystalline silver particles have been preparedwith a phenolic reducing compound and a polar protic solvent. See, forexample, U.S. Pat. No. 3,940,261 to Dannelly et al. In some cases,anisotropic metallic nanoparticles have been prepared with two differentreducing agents. See, for example, U.S. Pat. No. 8,030,242 to Uzio etal. and U.S. Pat. No. 8,652,232 to Bisson et al. In some cases,nanoparticles have been prepared with a hydrolysable gallotannin, suchas tannic acid. See, for example, U.S. Pat. No. 8,361,188 to Santhanamet al.

U.S. Pat. No. 8,613,888 to Whitcomb et al. and U.S. Patent PublicationNo. 2013/0340570 to Whitcomb et al. disclose metal ion reduction in thepresence of manganese or an ion of manganese. U.S. Pat. No. 8,613,888 toWhitcomb et al., U.S. Pat. No. 8,551,211 to Ollmann et al., U.S. PatentPublication No. 2013/0343950 to Ollmann et al., U.S. Patent PublicationNo. 2013/0340570 to Whitcomb et al., and U.S. Patent Publication No.2012/0294755 to Zhang et al. disclose metal ion reduction in thepresence of tin or an ion of tin.

SUMMARY

In some embodiments, a method is disclosed as comprising producing ametal product from a reaction mixture comprising a metal compoundcapable of forming a reducible metal ion, a protecting agent, a halidecompound capable of forming a halide ion or the halide ion produced fromthe halide compound, a first reducing agent, and a second reducingagent, where the second reducing agent comprises a phenol group. In someembodiments, the metal product comprises a metal seed particle. In someembodiments, the metal product comprises a metal nanowire.

In some embodiments, a first intermediate mixture comprises a firstportion of the metal compound, the protecting agent, the halide compoundor the halide ion produced from the halide compound, and the firstreducing agent; a second intermediate mixture comprises a second portionof the metal compound and the second reducing agent; and the firstintermediate mixture and the second intermediate mixture form thereaction mixture when combined.

In some embodiments, the reaction mixture comprises a reducible metalion produced from the metal compound. In some embodiments, the firstintermediate mixture comprises a first reducible metal ion produced fromthe first portion of the metal compound, and the second intermediatemixture comprises a second reducible metal ion produced from the secondportion of the metal compound.

In some embodiments, the second reducing agent comprises3,4-dihydroxybenzotriazole. In some embodiments, the second reducingagent comprises tannic acid. In some embodiments, the second reducingagent comprises 2,2′-isobutylidene-bis-(4,6-dimethyl-phenol). In someembodiments, the protecting agent comprises polyvinylpyrrolidone. Insome embodiments, the halide compound comprises cerium(III) chloride. Insome embodiments, the halide compound comprises manganese(II) chloride.In some embodiments, the first reducing agent comprises a polyol. Insome embodiments, the first reducing agent comprises propylene glycol.In some embodiments, the first reducing agent comprises ethylene glycol.

In some embodiments, a method is disclosed as comprising producing ametal product from a reaction mixture comprising a metal compoundcapable of forming a reducible metal ion, a protecting agent, a halidecompound capable of forming a halide ion or the halide ion produced fromthe halide compound, a polar aprotic solvent, and a first reducingagent, wherein the first reducing agent comprises a phenol group. Insome embodiments, the metal product comprises a metal seed particle. Insome embodiments, the metal product comprises a metal nanowire.

In some embodiments, the first reducing agent comprises3,4-dihydroxybenzotriazole. In some embodiments, the first reducingagent comprises tannic acid. In some embodiments, the first reducingagent comprises 2,2′-isobutylidene-bis-(4,6-dimethyl-phenol). In someembodiments, the protecting agent comprises polyvinylpyrrolidone. Insome embodiments, the halide compound comprises cerium(III) chloride. Insome embodiments, the halide compound comprises manganese(II) chloride.In some embodiments, the polar aprotic solvent comprises acetone. Insome embodiments, the polar aprotic solvent comprises acetonitrile.

In some embodiments, a method is disclosed as comprising producing ananoscale metal product from a reaction mixture comprising a metalcompound capable of forming a reducible metal ion, a protecting agent, ahalide compound capable of forming a halide ion or the halide ionproduced from the halide compound, and a first reducing agent, where thefirst reducing agent comprises a phenol group. In some embodiments, thenanoscale metal product comprises a nanoscale metal seed particle. Insome embodiments, the nanoscale metal product comprises a nanoscalemetal nanowire.

In some embodiments, the first reducing agent comprises3,4-dihydroxybenzotriazole. In some embodiments, the first reducingagent comprises tannic acid. In some embodiments, the first reducingagent comprises 2,2′-isobutylidene-bis-(4,6-dimethyl-phenol). In someembodiments, the protecting agent comprises polyvinylpyrrolidone. Insome embodiments, the halide compound comprises cerium(III) chloride. Insome embodiments, the halide compound comprises manganese(II) chloride.

DESCRIPTION OF FIGURES

FIG. 1 shows an optical micrograph of the reaction product using3,4-dihydroxybenzotriazole as a reducing agent after a total of 90minutes from when the solution AgNO₃ in PG was first added.

FIG. 2 shows an optical micrograph of the reaction product using3,4-dihydroxybenzotriazole as a reducing agent after a total of 120minutes from when the solution AgNO₃ in PG was first added.

FIG. 3 shows an optical micrograph of the reaction product using3,4-dihydroxybenzotriazole as a reducing agent after a total of 150minutes from when the solution AgNO₃ in PG was first added.

FIG. 4 shows an optical micrograph of the reaction product of FIG. 3after purification.

FIG. 5 shows a graph of the number distribution of silver nanowirediameters, in nanometers, taken from a random sample of 75 silvernanowires from five images of the reaction product after purification,such as that shown in FIG. 4.

FIG. 6 shows a graph of the number distribution of silver nanowirelengths, in μm, taken from a random sample of 347 silver nanowires fromnine images of the reaction product after purification, such as thatshown in FIG. 4.

FIG. 7 shows an optical micrograph of the reaction product using 75 mgof 2,2′-isobutylidene-bis-(4,6-dimethyl-phenol) as a reducing agentafter a total of 90 minutes from when the solution AgNO₃ in PG was firstadded.

FIG. 8 shows an optical micrograph of the reaction product using 75 mgof 2,2′-isobutylidene-bis-(4,6-dimethyl-phenol) as a reducing agentafter a total of 120 minutes from when the solution AgNO₃ in PG wasfirst added.

FIG. 9 shows an optical micrograph of the reaction product using 75 mgof 2,2′-isobutylidene-bis-(4,6-dimethyl-phenol) as a reducing agentafter a total of 150 minutes from when the solution AgNO₃ in PG wasfirst added.

FIG. 10 shows an optical micrograph of the reaction product of FIG. 9after purification.

FIG. 11 shows a graph of the number distribution of silver nanowirediameters, in nanometers, taken from a random sample of 75 silvernanowires from five images of the reaction product after purification,such as that shown in FIG. 10.

FIG. 12 shows a graph of the number distribution of silver nanowirelengths, in μm, taken from a random sample of 199 silver nanowires fromten images of the reaction product after purification, such as thatshown in FIG. 10.

FIG. 13 shows an optical micrograph of the reaction product using 101 mgof 2,2′-isolbutylidene-bis(4,6-dimethyl-phenol) after a total of 90minutes from when the solution AgNO₃ in PG was first added.

FIG. 14 shows an optical micrograph of the reaction product using 101 mgof 2,2′-isolbutylidene-bis(4,6-dimethyl-phenol) after a total of 120minutes from when the solution AgNO₃ in PG was first added.

FIG. 15 shows an optical micrograph of the reaction product using 101 mgof 2,2′-isolbutylidene-bis(4,6-dimethyl-phenol) after a total of 150minutes from when the solution AgNO₃ in PG was first added.

FIG. 16 shows a graph of the number distribution of silver nanowirediameters, in nanometers, taken from a random sample of 75 silvernanowires from five images of the reaction product, such as that shownin FIG. 15.

FIG. 17 shows a graph of the number distribution of silver nanowirelengths, in μm, taken from a random sample of 336 silver nanowires fromeight images of the reaction product, such as that shown in FIG. 15.

FIG. 18 shows a plot of silver ion concentration in millivolts over timein minutes as measured by a silver ion specific electrode for a reactionproduct using 0.96 g of 2,2′-isolbutylidene-bis(4,6-dimethyl-phenol).

FIG. 19 shows an optical micrograph of the purified reaction productfrom the reaction, which is plotted in FIG. 18.

FIG. 20 shows a graph of the number distribution of silver nanowirediameters, in nanometers, taken from image(s) of the reaction product,such as that shown in FIG. 19.

FIG. 21 shows a graph of the number distribution of silver nanowirelengths, in μm, taken from image(s) of the reaction product, such asthat shown in FIG. 19.

DESCRIPTION

All publications, patents, and patent documents referred to in thisdocument are incorporated by reference herein in their entirety, asthough individually incorporated by reference.

U.S. Provisional Application No. 62/020,431, filed Jul. 3, 2014,entitled “REDUCING AGENTS FOR SILVER MORPHOLOGY CONTROL,” is herebyincorporated by reference in its entirety.

Introduction

Silver nanowires (AgNW) are unique and useful silver structures thathave a wire-like shape in which the two short dimensions (the thicknessdimensions) are less than 300 nm, while the third dimension (the lengthdimension) is greater than 1 micron, preferably greater than 10 microns,and the aspect ratio (ratio of the length dimension to the larger of thetwo thickness dimensions) is greater than five. They are being used asconductors in electronic devices or as elements in optical devices,among other possible uses.

A common method of preparing nanostructures, such as, for example,nanowires, is the “polyol” process. Such a process is described in, forexample, Angew. Chem. Int. Ed. 2009, 48, 60, Y. Xia, Y. Xiong, B. Lim,S. E. Skrabalak, which is hereby incorporated by reference in itsentirety. In such processes, the polyol reduces a metal cation, such as,for example, a silver cation, to the desired metal nanostructureproduct, such as, for example, a silver nanowire.

Applicants have discovered that phenolic reducing agents having at leastone phenol group may be useful in preparing silver nanowires. A phenolicreducing agent may replace the “polyol” in the “polyol” process or usedas a second reducing agent in addition to the “polyol” that is eitherintroduced simultaneously with or subsequent to the polyol.

Preparation Methods and Materials

Silver seeds, which may lead to the formation of silver nanowires, maybe produced from a reaction mixture comprising at least one reducingagent, a protecting agent, a halide compound (or a halide ion producedfrom the halide compound), a metal salt, and an optional solvent. Insome embodiments, the reaction mixture may comprise one reducing agent,a protecting agent, a halide compound (or a halide ion produced from thehalide compound), a metal salt, and no other solvents. In such cases,the reducing agent may act as a solvent by being capable of dissolvingall other components of the reaction mixture into itself. In someembodiments, the reaction mixture may comprise one reducing agent, aprotecting agent, a halide compound (or a halide ion produced from thehalide compound), a metal salt, and at least one additional solvent,such as, for example, a polar protic or a polar aprotic solvent. In someembodiments, the reaction mixture may comprise a first reducing agent, asecond reducing agent, a protecting agent, a halide compound (or ahalide ion produced from the halide compound), and a metal salt. Thereaction mixture may be subjected to heating from about 100° C. to about200° C.

The reaction mixture may be formed from the combination of at least twointermediate mixtures, such as a first intermediate mixture and a secondintermediate mixture. The first intermediate mixture may be subjected toa reaction condition (e.g. N₂ headspace positive pressure at 0.5 L/min,heating at 110.0° C.±0.3° C., stirring at 200 rpm, selected residencetime, selected amount at which another substance has been added, etc.)prior to the second intermediate mixture being added to the firstintermediate mixture. The metal salt may be separated into a firstportion and a second portion into the first intermediate mixture and thesecond intermediate mixture, respectively, for combination into thereaction mixture.

In a first example, the metal salt and the halide compound (or thehalide ion produced from the halide compound) may be added in a laterstep to the mixture containing the at least one reducing agent, theprotecting agent, and the optional solvent. In such cases, the firstintermediate mixture comprises the at least one reducing agent, theprotecting agent, and the optional solvent, and the second intermediatemixture comprises the metal salt and the halide compound (or the halideion produced from the halide compound).

In a second example, a first portion of the metal salt may be added in alater step to the mixture containing the at least one reducing agent,the protecting agent, and the optional solvent, and the halide compound(or the halide ion produced from the halide compound) and a secondportion of the metal salt may be added after a certain amount (i.e. thefirst portion) of the metal salt has been added. In such cases, thefirst intermediate mixture comprises the at least one reducing agent,the protecting agent, and the optional solvent, the second intermediatemixture comprises a first portion of the metal salt, and the thirdintermediate mixture comprises a second portion of the metal salt andthe halide compound (or the halide ion produced from the halidecompound).

In a third example, the metal salt may be added in a later step to themixture containing the at least one reducing agent, the protectingagent, the halide compound (or the halide ion produced from the halidecompound), and the optional solvent. In such cases, the firstintermediate mixture comprises the at least one reducing agent, theprotecting agent, the halide compound (or the halide ion produced fromthe halide compound), and the optional solvent, and the secondintermediate mixture may comprise the metal salt.

In a fourth example, a second portion of the metal salt may be added ina later step to the mixture containing a first portion of the metalsalt, the at least one reducing agent, the protecting agent, the halidecompound (or the halide ion produced from the halide compound), and theoptional solvent. In such cases, the first intermediate mixturecomprises the second portion of the metal salt, and the firstintermediate mixture comprises the first portion of the metal salt, theat least one reducing agent, and the protecting agent, the halidecompound (or the halide ion produced from the halide compound), and theoptional solvent.

In a fifth example, a second portion of the metal salt and the halidecompound (or the halide ion produced from the halide compound) may beadded in a later step to the mixture containing a first portion of themetal salt, the at least one reducing agent, the protecting agent, andthe optional solvent. In such cases, the first intermediate mixturecomprises the second portion of the metal salt and the halide compound(or the halide ion produced from the halide compound), and the firstintermediate mixture comprises the first portion of the metal salt, theat least one reducing agent, and the protecting agent, and the optionalsolvent.

In a sixth example, a second portion of the metal salt may be added in alater step to the mixture containing a first portion of the metal salt,the at least one reducing agent, the protecting agent, and the optionalsolvent, and the halide compound (or the halide ion produced from thehalide compound) may be added after a certain amount of the secondportion of the metal salt has been added. In such cases, the firstintermediate mixture comprises the first portion of the metal salt, theat least one reducing agent, and the protecting agent, and the optionalsolvent, the second intermediate mixture comprises the second portion ofthe metal salt, and the third intermediate mixture comprises a thirdportion of the metal salt and the halide compound (or the halide ionproduced from the halide compound).

In cases where the reaction mixture comprises at least two reducingagents, the first reducing agent and the second reducing agent may beadded during the same or different steps. In a seventh example, thereaction mixture may comprise the first reducing agent, the metal salt,the protecting agent, the halide compound (or the halide ion producedfrom the halide compound), and the optional solvent, and the secondreducing agent. In an eighth example, the first intermediate mixture maycomprise the first reducing agent, a second reducing agent, a firstportion of the metal salt, the protecting agent, the halide compound (orthe halide ion produced from the halide compound), and the optionalsolvent, and the second intermediate may comprise a second portion ofthe metal salt. In a ninth example, the first intermediate mixture maycomprise the first reducing agent, a second reducing agent, a firstportion of the metal salt, the protecting agent, and the optionalsolvent, and the second intermediate may comprise a second portion ofthe metal salt and the halide compound (or the halide ion produced fromthe halide compound). In a tenth example, the first intermediate mixturemay comprise the first reducing agent, a first portion of the metalsalt, the protecting agent, the halide compound (or the halide ionproduced from the halide compound) and the optional solvent, and thesecond intermediate mixture may comprise the second reducing agent and asecond portion of the metal salt. In an eleventh example, the firstintermediate mixture may comprise the first reducing agent, a firstportion of the metal salt, the protecting agent, and the optionalsolvent, the second intermediate mixture may comprise the secondreducing agent and a second portion of the metal salt, and the thirdintermediate mixture may comprise the third portion of the metal saltand the halide compound (or the halide ion produced from the halidecompound).

Reducing Agents

Reducing agents are substances that have the ability to transfer theirelectrons to another substance. As discussed above, silver seeds may beprepared from a reaction mixture comprising at least one reducing agent.In exemplary embodiments, at least one of the reducing agents maycomprise a phenol group. In a first example, a reaction mixture maycomprise a first reducing agent comprising a polyol and a secondreducing agent comprising a phenol group. The polyol may be act as botha solvent to dissolve the metal salt (e.g. silver nitrate) to form themetal solution (e.g. silver solution) and as a reducing agent that iscapable of reducing the metal salt (e.g. silver nitrate) to metal (e.g.silver). In such cases, the second reducing agent may enhance thereducing capacity of the first reducing agent and/or participate as anadditional reducing agent. Non-limiting examples of polyols includeethylene glycol, glycerol, glucose, diethylene glycol, tri-ethyleneglycol, propylene glycol, butanediol, a dipropylene glycol, and/or apolyethylene glycol. The polyol may be a single polyol or a mixture oftwo or more different polyols (e.g. three, four, five, or more differentpolyols).

In a second example, a reaction mixture may comprise one reducing agentcomprising a phenol group. In this application, the term “phenolic”compound, “phenolic” reducing agent, or “phenol” group refers to acompound comprising at least one first aromatic ring, at least one firstoxygen atom, and at least one first hydrogen atom bonded to the at leastone first oxygen atom, where the at least one first aromatic ringcomprises at least one first carbon atom bonded to the at least onefirst oxygen atom. Non-limiting examples of phenolic reducing agentsinclude 3,4-dihydroxybenzotriazole,2,2′-isobutylidene-bis-(4,6-dimethyl-phenol), and tannic acid.

Reducible Metal Ions and Metal Products

Some embodiments provide methods comprising reducing at least onereducible metal ion to at least one metal. A reducible metal ion is acation that is capable of being reduced to a metal under some set ofreaction conditions. In such methods, the at least one first reduciblemetal ion may, for example, comprise at least one coinage metal ion. Acoinage metal ion is an ion of one of the coinage metals, which includecopper, silver, and gold. Or such a reducible metal ion may, forexample, comprise at least one ion of an IUPAC Group 11 element. Anexemplary reducible metal ion is a silver cation. Such reducible metalions may, in some cases, be provided as salts. For example, silvercations might, in some cases, be provided as silver nitrate.

In such embodiments, the at least one metal is that metal to which theat least one reducible metal ion is capable of being reduced to. Forexample, silver would be the metal to which a silver cation would becapable of being reduced to.

Metal Compounds

Some embodiments provide reaction mixtures comprising a metal compound.The metal compound can be any silver compound that produces metal whenreduced. The metal compound can be reduced by a polyol or a phenolicreducing agent. The metal compound can be a metal oxide, a metalhydroxide, or a metal salt (organic or inorganic).

Metal salts may dissociate in a solution into a metal cation and ananion. The metal cation may be reduced by a reducing agent to form ametal product, such as a silver seed, silver nanoparticle, or silvernanowire. The metal salt may be provided in various forms, such as, forexample, in solution, solid (e.g. solid powder), or as a suspension.Non-limiting examples of metal salts include nitrates, nitrites,sulfates, halides, carbonates, phosphates, azides, borates, sulfonates,carboxylates, substituted carboxylates, and salts and acids where themetal is part of an anion, or combinations thereof. Specificnon-limiting examples of metal salts include silver nitrate, silvernitrate, silver oxide, silver fluoride, silver acetate, or combinationsthereof.

Solvents

A solvent is a substance that dissolves a solute (a chemically differentliquid, solid, or gas) resulting in a solution. Solvents can beclassified as polar or non-polar based on their dielectric constant(e.g. relative static permittivity). The relative permittivity of amaterial under given conditions reflects the extent to which itconcentrates electrostatic lines of flux. Solvents with a dielectricconstant of less than 15 are generally considered non-polar. Polarsolvents, which have a relative static permittivity greater than 15, canbe further classified as protic and aprotic. Protic solvents are thosethat possess dissociable protons, while aprotic solvents lack suchdissociable protons. The Hansen δH parameters for protic solvents aregenerally greater than about 12, while those for aprotic solvents aregenerally less than about 12. Non-limiting examples of polar proticsolvents include formic acid, n-butanol, isopropanol, n-propanol,ethanol, methanol, acetic acid, nitromethane, and water. Non-limitingexamples of polar aprotic solvents include dichloromethane,tetrahydrofuran, ethyl acetate, acetone, dimethylformamide,acetonitrile, dimethyl sulfoxide, and propylene carbonate.

Protecting Agents

Some embodiments provide a reaction mixture comprising a protectingagent. Literature has suggested that the protecting agent may havedifferent purposes, such as avoiding particle sintering, capability ofabsorbing onto the metal nanostructure, etc. Generally, it is thoughtthat the protecting agent reduces or prevents direct contact betweenindividual nanostructures. Such literature has referred to “protectingagents” using other terms, such as “organic protective agent,”“protective agent,” “coordination compound,” “polymer capping agent,”“polymeric capping agent,” “capping agent,” “capping reagent,” and “softtemplate.” Use of the phrase “protecting agent” herein is intended toencompass all these other various phrases as well as other reactantsknown in the art that are added to the polyol synthesis of metalnanostructures to thereby reduce and/or prevent sintering oragglomeration.

Generally, the protecting agent should have minimal, if any, reactionwith other components in the reaction mixture. Nor should the protectingagent inhibit or prevent the solution mediation production of desirednanostructures, such as silver nanowires. The protecting agent may be asubstance capable of electronically interacting with a metal atom of ananoparticle. For example, the protecting agent may be capable of adative interaction with a metal atom on the surface of a nanoparticleand/or of chelating the metal atom. The protecting agent can compriseone or more atoms with one or more free electron pairs, such as, forexample, oxygen, nitrogen, and sulfur. The atoms with a free electronpair can be in the form of a functional group, such as, for example, ahydroxyl group, a carbonyl group, an ether group, an amino group, orcombinations thereof. Non-limiting examples of protecting agents thatcan be used alone or as mixtures include polyvinylpyrrolidone (PVP),polyvinyl alcohol, and surfactants, such as sodium dodecyl sulfate(SDS), larylamine and hydroxypropyl cellulose. In some embodiments, theprotecting agent is or comprises a substance that is capable of reducingthe metal compound. In such cases, the protecting agent may qualify as areducing agent.

Halide Compounds

A reaction mixture may comprise an optional halide compound that iscapable of forming halide ion(s). It is thought that various reactants(e.g. first reducing agent and protecting agent) may be contaminatedwith one or more halide compounds that are capable of forming halideion(s). Even with such halide contaminants occurring in the reactionmixture, the addition of optional halide compounds may still be requiredfor the reaction mixture to produce metal seeds that lead to theformation of the metal nanowires. Non-limiting examples of halidecompounds include cerium(III) chloride heptahydrate, managanese(II)chloride tetrahydrate, and tin(II) chloride dihydrate.

Nanostructures and Nanowires

In some embodiments, the metal product formed by such methods is ananostructure, such as, for example, a one-dimensional nanostructure.Nanostructures are structures having at least one “nanoscale” dimensionless than 300 nm, and at least one other dimension being much largerthan the nanoscale dimension, such as, for example, at least about 10 orat least about 100 or at least about 200 or at least about 1000 timeslarger. Examples of such nanostructures are nanorods, nanowires,nanotubes, nanopyramids, nanoprisms, nanoplates, and the like.“One-dimensional” nanostructures have one dimension that is much largerthan the other two dimensions, such as, for example, at least about 10or at least about 100 or at least about 200 or at least about 1000 timeslarger.

Such one-dimensional nanostructures may, in some cases, comprisenanowires. Nanowires are one-dimensional nanostructures in which the twoshort dimensions (the thickness dimensions) are less than 300 nm,preferably less than 100 nm, while the third dimension (the lengthdimension) is greater than 1 micron, preferably greater than 10 microns,and the aspect ratio (ratio of the length dimension to the larger of thetwo thickness dimensions) is greater than five. Nanowires are beingemployed as conductors in electronic devices or as elements in opticaldevices, among other possible uses. Silver nanowires are preferred insome such applications.

Such methods may be used to prepare nanostructures other than nanowires,such as, for example, nanocubes, nanorods, nanopyramids, nanotubes, andthe like. Nanowires and other nanostructure products may be incorporatedinto articles, such as, for example, electronic displays, touch screens,portable telephones, cellular telephones, computer displays, laptopcomputers, tablet computers, point-of-purchase kiosks, music players,televisions, electronic games, electronic book readers, transparentelectrodes, solar cells, light emitting diodes, other electronicdevices, medical imaging devices, medical imaging media, and the like.

Exemplary Embodiments

U.S. Provisional Application No. 62/020,431, filed Jul. 3, 2014,entitled “REDUCING AGENTS FOR SILVER MORPHOLOGY CONTROL,” which ishereby incorporated by reference in its entirety, disclosed thefollowing 35 non-limiting exemplary embodiments:

A. A method comprising:

-   -   producing a metal product from a reaction mixture comprising a        metal compound capable of forming a reducible metal ion, a        protecting agent, a halide compound capable of forming a halide        ion or the halide ion produced from the halide compound, a first        reducing agent, and a second reducing agent, wherein the second        reducing agent comprises a phenol group.        B. The method according to embodiment A, wherein the metal        product comprises a metal seed particle.        C. The method according to either of embodiments A or B, wherein        the metal product comprises a metal nanowire.        D. The method according to any of embodiments A-C,    -   wherein a first intermediate mixture comprises a first portion        of the metal compound, the protecting agent, the halide compound        or the halide ion produced from the halide compound, and the        first reducing agent,    -   wherein a second intermediate mixture comprises a second portion        of the metal compound and the second reducing agent, and    -   wherein the first intermediate mixture and the second        intermediate mixture form the reaction mixture when combined.        E. The method according to any of embodiments of A-D, wherein        the reaction mixture comprises a reducible metal ion produced        from the metal compound.        F. The method according to any of embodiments A-D, wherein the        first intermediate mixture comprises a first reducible metal ion        produced from the first portion of the metal compound, and the        second intermediate mixture comprises a second reducible metal        ion produced from the second portion of the metal compound.        G. The method according to any of embodiments A-F, wherein the        second reducing agent comprises 3,4-dihydroxybenzotriazole.        H. The method according to any of embodiments A-G, wherein the        second reducing agent comprises tannic acid.        J. The method according to any of embodiments A-H, wherein the        second reducing agent comprises        2,2′-isobutylidene-bis-(4,6-dimethyl-phenol).        K. The method according to any of embodiments A-J, wherein the        protecting agent comprises polyvinylpyrrolidone.        L. The method according to any of embodiments A-K, wherein the        halide compound comprises cerium(III) chloride.        M. The method according to any of embodiments A-L, wherein the        halide compound comprises manganese(II) chloride.        N. The method according to any of embodiments A-M, wherein the        first reducing agent comprises a polyol.        P. The method according to any of embodiments A-N, wherein the        first reducing agent comprises polyethylene glycol.        Q. The method according to any of embodiments A-P, wherein the        first reducing agent comprises ethylene glycol.        R. A method comprising    -   producing a metal product from a reaction mixture comprising a        metal compound capable of forming a reducible metal ion, a        protecting agent, a halide compound capable of forming a halide        ion or the halide ion produced from the halide compound, a polar        aprotic solvent, and a first reducing agent, wherein the first        reducing agent comprises a phenol group.        S. The method according to embodiment R, wherein the metal        product comprises a metal seed particle.        T. The method according to either of embodiments R or S, wherein        the metal product comprises a metal nanowire.        U. The method according to any of embodiments R-T, wherein the        first reducing agent comprises 3,4-dihydroxybenzotriazole.        V. The method according to any of embodiments R-U, wherein the        first reducing agent comprises tannic acid.        W. The method according to any of embodiments R-V, wherein the        first reducing agent comprises        2,2′-isobutylidene-bis-(4,6-dimethyl-phenol).        X. The method according to any of embodiments R-W, wherein the        protecting agent comprises polyvinylpyrrolidone.        Y. The method according to any of embodiments R-X, wherein the        halide compound comprises cerium(III) chloride.        Z. The method according to any of embodiments R-Y, wherein the        halide compound comprises manganese(II) chloride.        AA. The method according to any of embodiments R-Z, wherein the        polar aprotic solvent comprises acetone.        AB. The method according to any of embodiments R-AA, wherein the        polar aprotic solvent comprises acetonitrile.        AC. A method comprising:    -   producing a nanoscale metal product from a reaction mixture        comprising a metal compound capable of forming a reducible metal        ion, a protecting agent, a halide compound capable of forming a        halide ion or the halide ion produced from the halide compound,        and a first reducing agent, wherein the first reducing agent        comprises a phenol group.        AD. The method according to embodiment AC, wherein the nanoscale        metal product comprises a nanoscale metal seed particle.        AE. The method according to either of embodiments AC or AD,        wherein the nanoscale metal product comprises a nanoscale metal        nanowire.        AF. The method according to any of embodiments AC-AE, wherein        the first reducing agent comprises 3,4-dihydroxybenzotriazole.        AG. The method according to any of embodiments AC-AF, wherein        the first reducing agent comprises tannic acid.        AH. The method according to any of embodiments AC-AG, wherein        the first reducing agent comprises        2,2′-isobutylidene-bis-(4,6-dimethyl-phenol).        AJ. The method according to any of embodiments AC-AH, wherein        the protecting agent comprises polyvinylpyrrolidone.        AK. The method according to any of embodiments AC-AJ, wherein        the halide compound comprises cerium(III) chloride.        AL. The method according to any of embodiments AC-AK, wherein        the halide compound comprises manganese(II) chloride.

Further non-limiting exemplary embodiments include:

AM. A method comprising:

-   -   providing at least one reducing agent comprising at least one        phenol group, the at least one reducing agent not also        comprising a halogen atom, and    -   reducing at least one silver ion to at least one silver nanowire        in a reaction mixture comprising the at least one reducing        agent.        AN. The method according to embodiment AM, wherein the at least        one reducing agent comprises at least one of        3,4-dihydroxybenzotriazole,        2,2′-isobutylidene-bis-(4,6-dimethyl-phenol), and tannic acid.        AP. The method according to embodiment AM, wherein the at least        one reducing agent is selected from a group consisting of        3,4-dihydroxybenzotriazole,        2,2′-isobutylidene-bis-(4,6-dimethyl-phenol), and tannic acid.        AQ. The method according to any of embodiments AM-AP, wherein        the reaction mixture further comprises at least one polyol.        AR. The method according to embodiment AQ, wherein the at least        one polyol comprises propylene glycol.        AS. The method according to any of embodiments AM-AR, wherein        the reaction mixture further comprises at least one protecting        agent.        AT. The method according to embodiment AS, wherein the at least        one protecting agent comprises polyvinylpyrrolidone.        AU. The method according to any of embodiments AM-AT, wherein        the reaction mixture further comprises at least one halide        compound capable of forming a halide ion.        AV. The method according to any of embodiments AM-AU, wherein        the reaction mixture further comprises at least one halide ion.        AW. The method according to any of embodiments AM-AV, wherein        the reaction mixture further comprises at least one polar        aprotic solvent.        AX. The method according to embodiment AW, wherein the at least        one polar aprotic solvent comprises acetone.

EXAMPLES Materials

All materials used in the following examples are readily available fromstandard commercial sources, such as Sigma-Aldrich Co. LLC. (St. Louis,Miss.) unless otherwise specified. The following additional materialswere used.

Propylene glycol (PG) is available from BASF.

Polyvinylpyrrolidone (PVP) is available from BASF under the trade nameKOLLIDON®, such as KOLLIDON® 90 F.

2,2′ -isobutylidene-bis-(4,6-dimethyl-phenol), which has the structure

is available from Addivant (Middlebury, Conn.) under the trade nameLOWINOX® 221B46.

Instruments

A 0.5 L reaction flask with four necks was used to contain reactionmaterials. In our experiments, each of the four necks was used forinserting a stirring shaft, a condenser, a thermometer, or materials,such as reagents or nitrogen. The reaction flask is available fromChemglass Life Sciences.

A 12 gauge Teflon syringe needle was used to transfer materials from afirst container (e.g. bottle) to a second container (e.g. flask).

A polished glass stirring shaft having dimensions of 1 cm×40 cm withfour blades angled 15° having outer diameter of 4.2 cm was used to stirmaterials. The stirring shaft is available from Chemglass Life Sciences.

A VS-3003 multi-gas sampling unit with a VA-3002 multi-gas analyzer unitwith a nitrogen flow rate of 1.5 L/min and selected NO range of 0-2000ppm was used to quantitatively monitor for NO formation. The units areavailable from Horiba, Ltd.

Example 1

After overnight sparging, 250 mL of propylene glycol (PG), 68 mg of3,4-dihydroxybenzotriazole, and 4.5 g of polyvinylpyrrolidone (PVP) weresubjected to N₂ headspace positive pressure at 0.5 L/min, heating at110.0±0.3° C., and stirring at 200 rpm. 24.0 mL of a 1.0 M solution ofsilver nitrate (AgNO₃) in PG was added at a rate of 0.5 mL/min. After2.0 mL of the 1.0 M solution of AgNO₃ in PG was added, 10 mL of 14 mMsolution of cerium(III) chloride heptahydrate (CeCl₃·7H₂O) in PG wasadded at a rate of 0.5 mL/min. The reaction product had a small numberof silver nanowires among many silver particles.

Example 2

The reaction product was prepared in a manner similar to that describedin Example 1, except that 11.7 mg of 3,4-dihydroxybenzotriazole was usedinstead of 68 mg of 3,4-dihydroxybenzotriazole. Nitric oxide (NO) levelswere quantitatively monitored and determined to be relatively lowthroughout the reaction.

FIG. 1 shows an optical micrograph of the reaction product after a totalof 90 minutes from when the solution AgNO₃ in PG was first added. NOlevels were at 0 ppm. FIG. 2 shows an optical micrograph of the reactionproduct after a total of 120 minutes from when the solution AgNO₃ in PGwas first added. NO levels were at 4 ppm. FIG. 3 shows an opticalmicrograph of the reaction product after a total of 150 minutes fromwhen the solution AgNO₃ in PG was first added. NO levels were at 16 ppm.FIG. 4 shows an optical micrograph of the reaction product of FIG. 3after purification.

FIG. 5 shows a graph of the distribution of silver nanowire diameterstaken from a random sample of 75 silver nanowires from five images ofthe reaction product after purification, such as that shown in FIG. 4.For the silver nanowire diameters, the mean, median, standard deviation,minimum, and maximum were 60.24 nm, 59.55 nm, 11.21 nm, 40.01 nm, and97.23 nm, respectively. FIG. 6 shows a graph of the distribution ofsilver nanowire lengths taken from a random sample of 347 silvernanowires from nine images of the reaction product after purification,such as that shown in FIG. 4. For the silver nanowire lengths, the mean,median, standard deviation, minimum, and maximum were 9.62 μm, 7.81 μm,7.51 μm, 0.18 μm, and 37.02 μm, respectively.

Example 3

The reaction product was prepared in a manner similar to that describedin Example 2, except that 75 mg of2,2′-isobutylidene-bis-(4,6-dimethyl-phenol) replaced the 11.7 mg of3,4-dihydroxybenzotriazole.

FIG. 7 shows an optical micrograph of the reaction product after a totalof 90 minutes from when the solution AgNO₃ in PG was first added. NOlevels were at 0 ppm. FIG. 8 shows an optical micrograph of the reactionproduct after a total of 120 minutes from when the solution AgNO₃ in PGwas first added. NO levels were at 16 ppm. FIG. 9 shows an opticalmicrograph of the reaction product after a total of 150 minutes fromwhen the solution AgNO₃ in PG was first added. NO levels were at 20 ppm.FIG. 10 shows an optical micrograph of the reaction product of FIG. 9after purification.

FIG. 11 shows a graph of the distribution of silver nanowire diameterstaken from a random sample of 75 silver nanowires from five images ofthe reaction product after purification, such as that shown in FIG. 10.For the silver nanowire diameters, the mean, median, standard deviation,minimum, and maximum were 42.27 nm, 43.26 nm, 7.65 nm, 26.98 nm, and62.8 nm, respectively. FIG. 12 shows a graph of the distribution ofsilver nanowire lengths taken from a random sample of 199 silvernanowires from ten images of the reaction product after purification,such as that shown in FIG. 10. For the silver nanowire lengths, themean, median, standard deviation, minimum, and maximum were 6.85 μm,6.89 μm, 3.24 μm, 0.27 μm, and 27.18 μm, respectively.

Example 4

The reaction product was prepared in a manner similar to that describedin Example 3, except that 101 mg of2,2′-isobutylidene-bis(4,6-dimethyl-phenol) was used and the 4.5 g ofPVP was added after 2.0 mL of 1.0 M AgNO₃ in PG was added.

FIG. 13 shows an optical micrograph of the reaction product after atotal of 90 minutes from when the solution AgNO₃ in PG was first added.NO levels were at 0 ppm. FIG. 14 shows an optical micrograph of thereaction product after a total of 120 minutes from when the solutionAgNO₃ in PG was first added. NO levels were at 7 ppm. FIG. 15 shows anoptical micrograph of the reaction product after a total of 150 minutesfrom when the solution AgNO₃ in PG was first added. NO levels were at 11ppm.

FIG. 16 shows a graph of the distribution of silver nanowire diameterstaken from a random sample of 75 silver nanowires from five images ofthe reaction product, such as that shown in FIG. 15. For the silvernanowire diameters, the mean, median, standard deviation, minimum, andmaximum were 39.57 nm, 39.54 nm, 5.34 nm, 28.38 nm, and 49.73 nm,respectively. FIG. 17 shows a graph of the distribution of silvernanowire lengths taken from a random sample of 336 silver nanowires fromeight images of the reaction product, such as that shown in FIG. 15. Forthe silver nanowire lengths, the mean, median, standard deviation,minimum, and maximum were 8.76 μm, 8.37 μm, 3.8 μm, 1.21 μm, and 24.35μm, respectively.

Example 5

400 mL PG containing 5.55 g of PVP, 0.196 g of2,2′-isobutylidene-bis-(4,6-dimethyl-phenol), and 58.5 mg of MnCl₂·4H₂Owas sparged with N₂ overnight and then switched to positive headspacepressure prior to AgNO₃ addition. 24.0 mL of 1.0 M AgNO₃ in PG was addedto the mixture at 0.5 mL/min. The mixture was heated to 150° C.±0.5° C.and held at that temperature until the mV reading on a silver ionspecific electrode stabilized at which point the mixture was cooled. Thetotal amount of NO produced during reaction is about 2.2 mmol.

FIG. 18 shows a plot of silver ion concentration in mV over time inminutes as measured by a silver ion specific electrode as described inU.S. Provisional Application No. 62/170,164, filed Jun. 3, 2015, whichis hereby incorporated by reference. FIG. 19 shows an optical micrographof the purified reaction product. FIG. 20 shows a graph of thedistribution of silver nanowire diameters taken from image(s) of thereaction product, such as that shown in FIG. 19. FIG. 21 shows a graphof the distribution of silver nanowire lengths taken from image(s) ofthe reaction product, such as that shown in FIG. 19.

Example 6

To 280 mL of ethylene glycol (EG), 5.19 g of EG containing 53.0 mg AgNO₃and 106.5 mg of tannic acid and 2.1 g of 9.3 mM SnCl₂·2H₂O in EG wasadded. The base mixture was degassed with N₂ while being stirred at 100rpm for two hours. A solution of 0.25 M AgNO₃ in EG and 0.84 Mpolyvinylpyrrolidone (PVP) was degassed and heated to 145° C. for atleast 60 minutes. The solution was added to the base mixture 0.8 mL/minto create the reaction mixture. When cooled to ambient temperature, thereaction mixture is diluted by an equal volume of acetone andcentrifuged at 400 G for 45 minutes. The decanted solid was re-dispersedin 200 mL isopropanol (IPA), agitated for 10 minutes, centrifuged,decanted, and diluted with 15 mL of IPA. No silver nanowires weredetected.

The invention has been described in detail with reference to specificembodiments, but it will be understood that variations and modificationscan be effected within the spirit and scope of the invention. Thepresently disclosed embodiments are therefore considered in all respectsto be illustrative and not restrictive. The scope of the invention isindicated by the claims and all changes that come within the meaning andrange of equivalents thereof are intended to be embraced there.

What is claimed:
 1. A method comprising: providing at least one reducingagent comprising at least one phenol group, the at least one reducingagent not also comprising a halogen atom, and reducing at least onesilver ion to at least one silver nanowire in a reaction mixturecomprising the at least one reducing agent.
 2. The method according toclaim 1, wherein the at least one reducing agent comprises at least oneof 3,4-dihydroxybenzotriazole,2,2′-isobutylidene-bis-(4,6-dimethyl-phenol), and tannic acid.
 3. Themethod according to claim 1, wherein the at least one reducing agent isselected from a group consisting of 3,4-dihydroxybenzotriazole,2,2′-isobutylidene-bis-(4,6-dimethyl-phenol), and tannic acid.
 4. Themethod according to claim 1, wherein the reaction mixture furthercomprises at least one polyol.
 5. The method according to claim 4,wherein the at least one polyol comprises propylene glycol.
 6. Themethod according to claim 1, wherein the reaction mixture furthercomprises at least one protecting agent.
 7. The method according toclaim 6, wherein the at least one protecting agent comprises polyvinylpyrrolidone.
 8. The method according to claim 1, wherein the reactionmixture further comprises at least one halide compound capable offorming a halide ion.
 9. The method according to claim 1, wherein thereaction mixture further comprises at least one halide ion.
 10. Themethod according to claim 1, wherein the reaction mixture furthercomprises at least one polar aprotic solvent.
 11. The method accordingto claim 10, wherein the at least one polar aprotic solvent comprisesacetone.